In a transmitter such as a base station of a wireless communication system, the power consumption of the power amplifier that is provided in the last stage monopolizes 50% or more of the total power consumption of the transmitter. Switching amplifiers are therefore receiving attention in recent years as power amplifiers having high power efficiency.
On the other hand, from the standpoints of efficient utilization of frequency and economy, demand is increasing for the capability to amplify and transmit signals of a plurality of frequency bands and thus handle a plurality of bands in a transmitter. As a result, the latest transmitters also include transmitters that, by combining the above-described switching amplifiers and digital modulators that are capable of flexibly converting frequency characteristics, achieve both the capability to handle a plurality of bands, and further, an improvement of power efficiency.
FIG. 1 shows an example of the configuration of a transmitter that uses a digital modulator and switching amplifier. In the following figures including FIG. 1, numerical values including voltage values and resistance values shown in the figures are only examples and are not limited to these values.
In the transmitter shown in FIG. 1, quadrature-phase baseband signals I(t) and Q(t), that are generated in digital baseband signal generator 301, are converted to amplitude signal r(t) and phase signal θ(t) in converter 302.
Phase signal θ(t) is applied as input to converter 303 together with an amplitude signal whose value is fixed to “1”, and after being converted again to quadrature-phase baseband signals I(t) and Q(t), is up-converted in IQ modulator 304.
Delta-sigma modulator 305 uses the output signal of IQ modulator 304 as a clock signal to subject amplitude signal r(t) to delta-sigma modulation. Delta-sigma modulator 305 here carries out operation to supply multiple values when the carrier frequency is low (for example, 800 MHz), and in FIG. 1 supplies three values.
The output signals of delta-sigma modulator 305 are multiplied by the output signal of IQ modulator 304 in multiplier 306 and supplied to switching amplifier 307.
Corresponding to the multiple-value output of delta-sigma modulator 305, switching amplifier 307 includes the same number of switch elements and power sources as the number of values supplied from delta-sigma modulator 305. In FIG. 1, delta-sigma modulator 305 has a three-value output, and switching amplifier 307 therefore has three power sources (Vdd, Vdd/2, and GND) and three switch elements. These three switch elements are controlled so that only one switch element is ON, and the output terminals of the switch elements are connected together. The voltage of the power source that is connected to the switch element that is turned ON is thus supplied to BPF (Band-Pass Filter) 308. The output signal of multiplier 306 is accordingly amplified by one of the three voltage values.
After suppression of frequency components other than the desired frequency component (for example, the 2.14 GHz band) by BPF 308, the output signal of switching amplifier 307 is radiated into the air from antenna (load) 309.
The following description relates to the details of the configuration of a switching amplifier that is incorporated as switching amplifier 307 when delta-sigma modulator 305 has binary output in the transmitter shown in FIG. 1. In other words, the switching amplifier described below is of a configuration that supplies two voltage values: a high-side voltage value and a low-side voltage value.
The switching amplifier shown in FIG. 2 includes: high-side gate (first high-side gate) 100-1 and low-side gate (first low-side gate) 100-2; and high-side driver 200-1 and low-side driver 200-2 that drive high-side gate 100-1 and low-side gate 100-2, respectively (for example, refer to Patent Document 1).
High-side gate 100-1 is an n-channel FET. High-side gate 100-1 has its drain that serves as the power-source terminal, in which the drain is connected to power source 101-1 (power-source voltage: 30 V), and its source that serves as the output terminal, in which the source is connected to low-side gate 100-2. In addition, low-side gate 100-2 is an n-channel FET. Low-side gate 100-2 has its source that serves as the power-source terminal, in which the source is connected to ground, and its drain that serves as the output terminal, in which the drain is connected to high-side gate 100-1.
High-side driver 200-1 amplifies the output signal of multiplier 306 and applies the amplified signal as input to the gate that serves as the input terminal of high-side gate 100-1 to drive high-side gate 100-1. In addition, low-side driver 200-2 amplifies the output signal of multiplier 306 and applies the amplified signal as input to the gate that serves as the input terminal of low-side gate 100-2 to drive low-side gate 100-2.
High-side gate 100-1 and low-side gate 100-2 are controlled by high-side driver 200-1 and low-side driver 200-2 such that one turns ON and the other turns OFF. For example, power-source voltage 30V of power source 101-1 is supplied if high-side gate 100-1 is ON, and the ground voltage of ground is supplied if low-side gate 100-2 is ON.
High-side gate 100-1 and low-side gate 100-2 are assumed to be depletion-type FETs in which the potential across the gate and source is made the same in order to turn ON and the potential across the gate and source is made −5V in order to turn OFF. As a result, the output voltage of high-side driver 200-1 must be made 30V and −5V in order to turn ON and OFF, respectively, high-side gate 100-1, and the output voltage of low-side driver 200-2 must be made 0V and −5V in order to turn ON and OFF, respectively, low-side gate 100-2.
As shown in FIG. 3, high-side driver 200-1 and low-side driver 200-2 usually adopt a construction in which a resistor and a switch element are interposed between two power sources.
More specifically, high-side driver 200-1 is of a configuration in which resistor 203-1 (2Ω) and internal amplifier element 201-1 that is an n-channel FET are interposed between power source 202-1 (power-source voltage: −5V) and power source 204-1 (power-source voltage: 30V); and the drain that serves as the output terminal of internal amplifier element 201 is connected to the gate that serves as the input terminal of high-side gate 100-1.
On the other hand, low-side driver 200-2 is of a configuration in which resistor 203-2 (2Ω) and internal amplifier element 201-2 that is an n-channel FET are interposed between power source 202-2 (power-source voltage: −5V) and power source 204-2 (power-source voltage: 0V); and the drain that serves as the output terminal of internal amplifier element 201-2 is connected to the gate that serves as the input terminal of low-side gate 100-2.
The operation of the switching amplifier shown in FIG. 3 is next described with reference to FIG. 4.
(A) Operation when Supplying High-Side Voltage
The operation when supplying high-side voltage (i.e., power-source voltage 30V of power source 101-1) is first described.
When high-side voltage is to be supplied, high-side gate 100-1 is turned ON and low-side gate 100-2 is turned OFF.
For this purpose, in high-side driver 200-1, internal amplifier element 201-1 is turned OFF, and the potential across the gate and source of high-side gate 100-1 is made the same to turn ON high-side gate 100-1. In this state, current does not flow in resistor 203-1, whereby the power consumption in high-side driver 200-1 ideally becomes zero.
In low-side driver 200-2, on the other hand, internal amplifier element 201-2 is turned ON, and the potential across the gate and source of low-side gate 100-2 is made −5V to turn OFF low-side gate 100-2. In this state, current flows in resistor 203-2. At this time, the voltage drop at resistor 203-2 is 5V, and the current value becomes 2.5 A. As a result, power consumption of 12.5 W occurs instantaneously. However, considering that the power output of a macro-base station is in the order of 20 W, the power consumption in low-side driver 200-2 is suppressed to a low level.
(B) Operation when Supplying Low-Side Voltage
The operation when supplying low-side voltage (i.e., the ground voltage of the Ground) is next described.
When low-side voltage is to be supplied, low-side gate 100-2 is turned ON and high-side gate 100-1 is turned OFF.
For this purpose, in low-side driver 200-2, internal amplifier element 201-2 is turned OFF, and the potential across the gate and source of low-side gate 100-2 is made the same to turn ON low-side gate 100-2. In this state, current does not flow to resistor 203-2, whereby the power consumption in low-side driver 200-2 ideally becomes zero.
In high-side driver 200-1, on the other hand, internal amplifier element 201-1 is turned ON, and the potential across the gate and source of high-side gate 100-1 is made −5V to turn OFF high-side gate 100-1. In this state, current flows to resistor 203-1. At this time, the voltage drop at resistor 203-1 is 35V, and the current value becomes 17.5 A. As a result, the power consumption in high-side driver 200-1 instantaneously far surpasses the output power of the macro-base station and undergoes extreme increase that exceeds 600 W.
Methods of reducing the power consumption in resistor 203-1 include a method of simply increasing the resistance value of resistor 203-1, but this method results in the increase of the RC product realized by resistor 203-1 and the capacitance of high-side gate 100-1 in the following stage, whereby high-speed operation becomes impossible.
Thus, in order to maintain high-speed operation, the resistance value of resistor 203-1 must be reduced in the order of 2Ω.